1. Field of the Invention
This invention relates to a process for preparing a methylphenol, and more particularly, to a process for preparing a methylphenol by acid-decomposing the hydroperoxide obtained by oxidizing an alkylbenzene havine one secondary alkyl group and 1-3 methyl groups.
2. Description of the Prior Art
A process for preparing phenol by oxidizing cumene to the hydroperoxide, acid-decomposing the resulting hydroperoxide in the presence of an acid catalyst such as a mineral acid to produce phenol and acetone, neutralizing the acid catalyst and separating phenol and acetone by distillation is known as the cumene process.
The cumene process has various drawbacks when the process is directly applied to a process for producing a methylphenol and ketone starting from an alkylbenzene having one secondary alkyl group and 1-3 methyl groups. When a methylbenzene having one secondary alkyl group and 1-3 methyl groups are oxidized, there are formed two or more hydroperoxides such as a tertiary hydroperoxide (hereinafter referred to as "3 - HPO") formed by oxidation of the tertiary carbon atoms of the secondary alkyl group and a primary hydroperoxide (herinafter referred to as "1 - HPO") formed by oxidation of the primary carbon atom of the methyl group directly attached to benzene nucleus, and therefore, the subsequent acid decomposition reaction becomes complicated.
The ratio of the resulting 3 - HPO to 1 - HPO varies depending upon the type of the secondary alkyl group of the alkylbenzene and number and position of the methyl groups directly attached to the benzene nucleus. For example, oxidation of 3,5-dimethylcumene gives about 70 molar parts of 3,5-dimethyl-.alpha.,.alpha.-dimethylbenzyl hydroperoxide (as 3 - HPO) and about 30 molar parts of 3-methyl-5-isopropylbenzyl hydroperoxide (as 1 - HPO).
As the result of the acid decomposition treatment, most of the 3 - HPO is acid-decomposed to form a methyl phenol and acetone, and small amount of 3 - HPO is converted to alcohols and other by-products. On the other hand, most of the 1 - HPO is acid-decomposed to form an alkylphenol having one secondary alkyl group and formaldehyde, and a small amount of 1 - HPO is converted to alcohols and other by-products. The acid decomposition reaction rate of 1 - HPO is far less than that of 3 - HPO and therefore, as the acid decomposition reaction proceeds the, 3 - HPO concentration decreases faster and finally the residual hydroperoxide is mostly 1 - HPO. In order to decompose the residual 1 - HPO, a severer reaction condition should be applied.
When an oxidation product solution containing 70 molar parts of 3 - HPO and 30 molar parts of 1 - HPO obtained by oxidation of 3,5-dimethylcumene was acid-decomposed to the hydroperoxide conversion of 90%, 99.0% by mole of the residual hydroperoxide was 1 - HPO, i.e. 3-methyl-5-isopropylbenzyl hydroperoxide.
Further, alcohols derived from 3 - HPO and 1 - HPO cause condensation reactions with 3 - HPO and/or 1 - HPO during the acid decomposition treatment to form diaralkyl peroxides. The resulting diaralkyl peroxides do not react any further under the usual acid decomposition conditions. An especially severe reaction condition is necessary to proceed the reaction further.
It is very difficult to recover the desired methylphenol by neutralizing the acid decomposition product solution obtained under usual conditions and containing residual hydroperoxides mainly composed of 1 - HPO and diaralkyl peroxides, and then distilling from the resultant solution. For example, when the acid decomposition product solution is heated, undesirable side reactions are caused by these peroxides to produce many by-products each having a boiling point close to that of the desirable methylphenol resulting in lower yield and purity of the methylphenol, and coloring thereof. Further the high content of the peroxides may result in an accidental explosion upon distillation. Therefore, for the purpose of facilitating the separation of the methylphenol by distillation, it is desirable to obtain an acid decomposition product solution containing as small an amount of the residual hydroperoxides as possible by sufficiently carrying out the acid composition reaction. However, when the acid decomposition treatment is conducted under severer conditions such as higher temperature and longer reaction time so as to reduce the content of residual hydroperoxides composed mainly of 1 - HPO and diaralkyl peroxide, condensation reaction of formaldehyde produced by the acid decomposition of 1 - HPO with the desired methylphenols is caused and high boiling point products are formed and, in addition, side reactions due to the peroxides occur so that the yield and/or the purity of the methylphenol are lowered.
For example, an oxidation product solution containing 70 molar parts of 3 - HPO and 30 molar parts of 1 - HPO obtained by oxidizing 3,5-dimethylcumene was acid-decomposed in solvent acetone in the presence of perchloric acid as a catalyst and the acid decomposition treatment was stopped when 84.96% of the hydroperoxide was decomposed. The resulting acid decomposition product solution was neutralized and analyzed by gas chromatography. An example of the chromatograms is shown in FIG. 1, which indicates that the neutralized solution contains six by-products as well as 3,5-xylenol and acetone. When the neutralized solution was distilled under reduced pressure in a batch-wise system by a packed tower of 20 theoretical plates, the separation of 3,5-xylenol from the X 4 component was very difficult because their boiling points are so close. In addition, thus obtained 3,5-xylenol was markedly colored and when the temperature of the still reaches 120.degree. C., some components were thermally decomposed and the decomposition products contaminate the end product, 3,5-xylenol. Therefore, it was actually impossible to obtain 3,5-xylenol of high purity in good yield.
In view of the foregoing, in order to obtain the methylphenol of high purity in good yield, it is necessary to stop the acid decomposition reaction when the most amount of 3 - HPO is acid-decomposed and then to decompose the residual hydroperoxides mainly composed of 1 - HPO and diaralkyl peroxides present in the acid decomposition product solution by a certain means prior to the distillation. As a method for solving such problem, Japanese Patent Publication No. 45854/1974 discloses a process for producing both cresol and acetone by decomposing cymene hydroperoxide obtained by oxidizing cymene with molecular oxygen which comprises decomposing cymene hydroperoxide in the presence of an acid catalyst, stopping the decomposition reaction when the concentration of the hydroperoxide in the reaction product falls in 0.5-10% by weight, neutralizing the reaction product, decomposing the residual hydroperoxides by heating the product and then recovering cresol and acetone. According to the above mentioned process, however, relatively long treating period and high temperature are necessary to thermally decompose the diaralkyl peroxides present in the acid decomposition product solution to such an extent that an explosive accident in the later step, e.g. a separation step of the desired methylphenol, can be avoided. Further, these requirements will result in lowering the yield of the methylphenol and the selectivity thereto.